const kNumKeys = 49;
const kFirstKey = 'C2';
const kNotes = ['C', 'C#', 'D', 'Eb', 'E', 'F', 'F#', 'G', 'G#', 'A', 'Bb', 'B'];
const kA0Frequency = 27.5;
const kSemiToneMultiplier = Math.pow(2, 1/12);

const audioCtx = new AudioContext();

function playNote(frequency, duration) {
    let currentGain = 1.0;

    const bassBoost = f => {
        const t = f / 20000;
        const a = 4.0;
        const b = 0.2;

        return b * t + a * (1 - t);
    };

    while (frequency < 20000) {
        const oscillator = new OscillatorNode(audioCtx);
        const gain = new GainNode(audioCtx);

        oscillator.type = 'sine';
        oscillator.frequency.value = frequency; // value in hertz
        oscillator.connect(gain);
        gain.gain.setValueAtTime(0, audioCtx.currentTime);
        gain.gain.exponentialRampToValueAtTime(bassBoost(frequency) * currentGain, audioCtx.currentTime + 0.010);
        gain.gain.linearRampToValueAtTime(1e-10, audioCtx.currentTime + 0.500);
        gain.connect(audioCtx.destination);
        oscillator.start();

        setTimeout(() => {oscillator.stop();}, duration);

        frequency *= 2;
        currentGain *= 0.3;
    }
}

function noteFrequency(key) {
    let [note, octave, ] = key.split(/(\d)/);
    octave = parseInt(octave);

    const interval = kNotes.indexOf(note) - kNotes.indexOf('A') + octave * 12;

    let freq = kA0Frequency * Math.pow(kSemiToneMultiplier, interval);

    return freq;
}

function onKeyClick(event) {
    const note = event.srcElement.id;
    const freq = noteFrequency(note);
    playNote(freq, 500);
}

function makeKey(note) {
    const key = document.createElement('span');
    if (note.includes('#') || note.includes('b')) {
        key.classList.add('blackKey');
    } else {
        key.classList.add('whiteKey');
    }
    key.id = note;
    key.addEventListener('click', onKeyClick);

    return key;
}

function setupKeyboard() {
    const keyboard = document.getElementById('keyboard');
    keyboard.innerHTML = '';

    let curIndex = kNotes.indexOf(kFirstKey.charAt(0));
    let octave = parseInt(kFirstKey.charAt(1));

    for (let i = 0; i < kNumKeys; i++) {
        const note = kNotes[curIndex] + octave;
        const key = makeKey(note)
        keyboard.appendChild(key);

        curIndex += 1;
        if (curIndex >= kNotes.length) {
            octave += 1;
            curIndex = curIndex % kNotes.length;
        }
    }
}

window.addEventListener('load', e => {
    setupKeyboard();
});